It is a larval fish. It is unusual for larvae to be pigmented, usually they are pretty much translucent. It doesn't have any major distinctive features so hard to ID any further.

When I was doing long night dives for coral spawning in September the water was full of fish larvae and post larvae. While waiting for the coral to go off I watched a group of about 10 post-larval surgeonfish settle.

Come on, Alex! I threw you an easy one. I thought for sure you'd know this one... It's no other than the post-larval stage of a whale shark.. Can't you see the resemblance?.. No, neither can I.

I think the critter's pigmentation was what truly caught my eye. Where as mostly everything I photograph in The Plankton Zone (15ft) is translucent, this little critter was a very bright yellow color. If I ever see one again I will attempt to get a better (closer) photograph.